The present invention relates to a dry etching method and apparatus fit for implementing the so-called gas chopping or time modulated etching method for carrying out plasma etching alternately in different gaseous atmospheres by periodically alternating between deposition and etching gases as a discharge processing gas.
With the miniaturization and integration of semiconductor LSIs, there has developed a demand for extremely high precision etching techniques at a submicron millimeter level during the process of manufacture. In microminiature processing of this sort, the aspect ratio d/r, i.e. the ratio of an aperture diameter r to depth d to be formed tends to increase. The higher the aspect ratio, the lower the etching speed becomes. Moreover, the processing shape also changes with the pattern size.
In microminiature processing of this sort, the present inventor has proposed a method of periodically conducting SF.sub.6 and CCl.sub.4 plasma etching to reduce the dependence of the etching speed on the aspect ratio; more specifically, the method proposed comprises the step of alternating between SF.sub.6 as an etching gas and CCl.sub.4 as a deposition gas sequentially at intervals of several seconds using a microwave plasma etching apparatus. This method is known as the so-called gas chopping or time modulated (TM) etching method since it premises the switching of the gas seed in terms of time and has been found noticeable in that the effect of reducing the dependence of the etching speed and the resulting shape on the aspect ratio as compared with plasma etching carried out by mixing these gases simultaneously. Those techniques have already been discussed in a collection of preparatory papers for the 35th applied-physics-related joint lecture, p 28 and following, Mar. 28, 1988 and the 18th (1986 International) Conference on Solid State Devices and Materials, Tokyo, 1986, pp 229-232.
The prior art TM etching method has certainly contributed to reducing the dependence of the etching speed and the resulting shape on the aspect ratio. Although difference in etching speed is on the decrease, depending on the size of the pattern to be formed, this fact is only applicable to a case where the gap between adjoining two patterns is 0.5 .mu.m or thereabout. However, etching accuracy for practical use has not yet been attained in the case of microminiature etching with the gap being not greater than 0.3 .mu.m. In other words, the problem is that a phenomenon generally called the micro-loading effect causes the difference in etching speed between narrow and wide etching portions. Patterns having small and large gaps are simultaneously produced by etching while minimizing the difference in etching speed between the gaps. The important technical task is therefore to establish a practical dry etching method and apparatus by which the etching is carried out while abating the aforementioned micro-loading effect.